Transmission lines provide guided transmission of power or information. Because power flows in loops, a transmission line requires two conductors. Two types of transmission lines commonly used within integrated circuits are microstrip transmission lines and coplanar waveguides. A microstrip transmission line includes a signal line, which is a conductive metal strip, and a planar ground separated from the signal line by dielectric. A coplanar waveguide includes a signal line flanked by two return lines. The return lines provide a return path for current flowing through the signal line.
The coplanar waveguide has more tunable parameter than the microstrip transmission line and can generally provide superior performance for that reason. Integration and process limitations constrain parameters other than lateral dimensions. With a microstrip transmission line, the width of the signal line can be varied but the thickness of the signal line and the dielectric are largely constrained. With a coplanar wave guide, the widths of the return lines and their spacing from the signal line are additional parameters that can be varied to achieve desired transmission line characteristics. The main disadvantage of the coplanar waveguide in comparison to the microstrip transmission line is that the coplanar waveguide requires a much greater amount of chip area.